1. Field of the Invention
The present invention relates to a free-vibration test method for examining building and civil structures having a natural vibration frequency more than 2.5 Hz using alternating human vibration-exciting method, and more particularly to a method for examining buildings and civil structures, wherein two sets of human subjects alternately act together to induce vibration in the structure, while synchronizing their actions to alternating-beat sounds generated from an electronic beat-sound generator such as a metronome, thereby causing the resonance of the structure, and thereafter the action is brought to a stop for obtaining a correct free-vibration waveform of the structure, thereby estimating a damping ratio of the structure.
2. Description of the Related Art
Due to vibrations caused by traffic, installed equipments, and construction works, even low-rise reinforced concrete structures are inevitably subject to a vibration problem that affects the serviceability of the structure.
In connection with a horizontal vibration, it is essential to correctly estimate the vibration characteristics of the structure, such as natural vibration frequency and damping ratio, to determine the serviceability of the structure.
That is, when examining the serviceability of a building, it is very important to measure the natural vibration frequency and the damping ratio of the structure. The natural vibration frequency is a very important variable for calculating the response acceleration of the structure by a related structure standard or a wind tunnel test. The response acceleration decreases in proportion to the increase of the natural vibration frequency. On the other hand, the damping ratio can be obtained only via measurement.
There are a number of vibration exciting methods that must be performed in advance for obtaining the damping ratio. In the methods, a force is repeatedly applied to a structure at a frequency corresponding to its natural period, whereby the response amplitude of the structure is increased in accordance with a resonance phenomenon. In one of such vibration exciting methods,human subjects are used as means for exciting the resonant vibration, which is called a human vibration-exciting test method.
In the human vibration-exciting test method, the vibration exciting actions are stopped at a time when the vibration amplitude reaches a sufficient value, and the subsequent free vibration of the structure is measured. In order to obtain an amplitude of more than a micro-tremor level, it is necessary to match the repeated period of the vibration exciting action as accurately as possible to the natural period of the structure, and also to provide a sufficient number of repeating times of the vibration excitation action.
In such a vibration excitation test method, the inertial force obtained by a movement of the weight of the human subjects is used as means for pushing the structure, with no need to prepare a special machine, thereby making it easy to perform the vibration test even just before the completion of the construction.
In more detail, in the human vibration-exciting test method, the free vibration waveform of a structure is obtained in such a way that, as shown in FIG. 1, human subjects, usually two to three, or, up to twenty if needed, act together to repeatedly apply a vibration exciting force on a wall in a top floor of the structure at a frequency corresponding to the natural vibration frequency of the structure, and stop the actions at a predetermined later time. However, only when the structure has a natural frequency in the range of 0.5 to 2.5 Hz can the human subjects repeatedly apply a sufficient vibration-exciting force at a frequency in the same range. That is, such a human vibration-exciting method is not effective in obtaining the free vibration waveform in the case of the natural frequency more than 2.5 Hz, because it is difficult for the human subjects to actually exert a sufficient vibration-exciting force 2.5 times per second.
Generally, the damping ratio (Y) is estimated using a logarithmic decrement method, which is expressed by an equation, Y=xc2xdxcfx80ln(X1/X1+2)xc3x97100, and illustrated by a graph showing the free vibration waveform in FIG. 2, where X and Y axes represent time and acceleration, respectively. The amplitude of the acceleration is 2 m/s2 at maximum, and gradually reduced as time goes on, where X1 represents the sum of positive amplitude of the first waveform and the absolute value of a negative amplitude of the second waveform.
That is, X1 represents the sum of a positive amplitude of an ith peak and the absolute value of a negative amplitude of an (i+1)th peak. The reason why the logarithmic decrement method is obtained using the positive amplitude in such a manner is that it is difficult to perform zero adjustment accurately in a general vibration measurement.
Meanwhile, the free vibration test can be performed using a vibration-exciting device in the prior art. However, this device is large in scale and requires space and labor to install it.
In addition, most vibration-exciting devices are expensive and complicated in terms of configuration, and are heavy so that it is very difficult to carry them to a top floor of the structure. Further, while the device excites a vibration, the rotation frequency of a weight of the device increases or decreases with time. Therefore, it is difficult to excite the vibration at a constant frequency. Furthermore, even after power is turned off to stop the vibration excitation, the weight of the device continues its rotation once or twice more, affecting the free vibration waveform.
On the other hand, the free vibration waveform can be obtained through micro-tremor measurement, but this approach requires an enormous amount of measurement data collected for several days. In addition, the micro-tremor has a low vibration amplitude. Therefore, in consideration of the characteristics of the damping ratio such that the damping ratio increases as the vibration amplitude increases, it is difficult to obtain a valid damping ratio in terms of the serviceability of the structure.
FIG. 3 is a view illustrating a tensioned steel-wire cutting method, a kind of vibration exciting method. In this method, both ends of a steel wire 11 are fixed to the ground and the top of a structure 10, and, after the steel wire is tensioned, the steel wire 11 is cut to induce a free vibration of the structure 10. However, it is difficult to apply this method to an actual structure, because it needs a sufficient space around the structure.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a method for examining structures having high natural vibration frequency using alternating human vibration-excitation, wherein the free vibration waveform of a building or civil structure having a natural vibration frequency more than 2.5 Hz, which corresponds to the upper limit of the range of frequencies at which a human vibration excitation is effective in the prior art, is obtained in such a manner that two sets of human subjects alternately and repeatedly exert a vibration exciting force on the structure at a frequency corresponding to half of the natural vibration frequency of the structure, so as to induce its vibration.
In accordance with the present invention, the above and other objects can be accomplished by the provision of an alternating human vibration-exciting method for obtaining a free vibration waveform of buildings or civil structure having a natural vibration frequency more than 2.5 Hz, wherein the free vibration waveform is obtained in such a manner that (a) first and second same-number sets of subjects having weights similar to each other alternately apply exciting forces to a wall on a top of the structure, (b) an electronic beat-sound generator is adjusted to a two-beat node to repeatedly generate two alternating beat sounds so that the first and second sets of human subjects can apply vibration exciting forces repeatedly at a frequency corresponding to half of the natural vibration frequency of the structure obtained by micro-tremor measurement, while alternately synchronizing their actions to the two alternating beat sounds, respectively, and (c) the vibration exciting action is stopped after the structure is brought to a sufficient resonance state, thereby obtaining a free vibration waveform.